The present invention relates generally to a valve timing adjusting mechanism for an internal combustion engine, which adjusts the timing of intake valve and/or exhaust valve actuation in accordance with the engine operating conditions. More specifically, the invention relates to a valve timing adjusting mechanism which is applicable not only to chain-drive-type timing systems but also to belt-drive-type timing systems.
As is well known, adjusting the intake valve timing and/or exhaust valve timing depending upon engine operating conditions, such as engine speed, engine load and so forth, helps optimize engine operation. For the sake of realizing engine operating condition-dependent valve timing control, a Timing Variator For The Timing System of Reciprocating Internal Combustion Engine has been proposed in U.S. Pat. No. 4,231,330, issued to Garcea, on Nov. 4, 1980. The disclosed timing variator hydraulically controls the angular position of intake valve and/or exhaust valve driving camshaft relative to a timing gear driven by the engine in synchronism with engine revolution.
On the other hand, in recent years, there has been a trend in the market toward lighter internal combustion engines for better vehicle drivability and for better fuel economy. In order to answer this requirement, belt-drive-type timing systems which significantly reduce the weight of the engine have been developed and put on the market. These belt-drive timing systems are known to create less noise than conventional chain-drive timing systems.
The timing variator of the aforementioned United States Patent is not applicable to such belt-drive timing systems since the working fluid, specifically engine lubricant used to actuate the valve timing would affect the timing belt and/or engagement between the timing belt and the timing gear. Specifically, in the timing variator of the United States Patent, the working fluid would react with material components of the belt and shorten its lifetime. Moreover, the working fluid tends to cause the belt to slip and thus adversely affects timing control.